It is known that the petroleum industry routinely uses conversion processes, and particularly cracking processes, in which hydrocarbon molecules with a high molecular weight and a high boiling point are broken down into smaller molecules with a lower boiling point suitable for the desired purpose.
Many of these processes employ fluid-bed conversion techniques in which solid particles (whether catalytic or not) are contacted for a very short time with hydrocarbons. The solid particles provide, in particular, the heat required for the conversion reaction.
The process most widely used at present is the so-called Fluid Catalytic Cracking, or FCC, process. However other fluid-bed conversion processes, and particularly thermal cracking or visbreaking processes, have been developed.
For the sake of simplicity, the invention will be described in this specification by way of example with reference to the catalytic cracking process. However, it should be understood that the invention is also applicable to most other fluid-bed hydrocarbon conversion processes in which the feedstock to be cracked is contacted in the vapor phase with solid particles, whether catalytic or not.
The most recent developments have confirmed that the most important parameters of the cracking reaction are the rapidity and homogeneity with which the feedstock is contacted with the catalyst particles, as well as the quality of the atomization and of the vaporization of the feedstock in the reaction zone of the tubular reactor.
The many studies conducted by the applicants and their assignee with a view to improving the heat transfer between the solid particles in the fluid bed and the feedstock to treated have (1) shown that the yields actually obtained in the highest efficiency cracking units used up to now have been lower than the yields to be expected on the basis of theoretical studies and (2) that this difference has been due mainly to poor distribution of the catalyst particles in the injection zone.
A first approach to a solution to this problem has already been proposed by the assignee's European Patent No. 191,695) and has sought to correct mainly the axial irregularities in the stream of hot catalyst coming from the regeneration zone. However, measurements made in particular on models have not only shown that the distribution of the catalyst particles was not uniform in a plane normal to the direction of the forward movement of the particles (also known as the radial distribution)--the concentration of these particles being greater at the periphery of the tubular reactor than in its center--but a downward backflow of solids was observed at the level of the walls of the reactor. The result is a decrease in the probability and quality of a collision between the atomized feedstock (generally injected in the more axial direction of the reactor) and the catalyst particles, since the latter are dispersed largely at the periphery.